The present invention relates to a seat back frame assembly of a vehicle seat, and more particularly to a reinforced framework of a frame assembly.
U.S. Pat. No. 4,796,954 discloses a seat back frame assembly of a vehicle seat. The frame assembly comprises a framework constituted by side frame members extending in a vertical direction, an upper frame member connecting upper ends of the side frame members and a lower frame member connecting lower ends of the side frame members, and zigzag springs provided between the side frame members.
The said first conventional assembly has an advantage capable of being formed light and an advantage capable of providing an improved comfort. However, the conventional assembly has a disadvantage of providing a low torsional rigidity. A torsional external force is strongly applied to a bonding portion between the side frame members and the lower frame member. For example, when a child sits on an armrest mounted to the side frame member, the bonding portion has to support the side frame member so as not to bend toward an front outer side with respect to the lower frame member.
U.S. Pat. No. 5,044,693 discloses a seat back frame assembly having a bracket mounted to the side frame member. However, the bracket of the second conventional assembly has no function of reinforcing the bonding portion between the side frame member and the lower frame member.
U.S. Pat. No. 5,193,881 discloses a seat back frame assembly provided with a center plate member, side plate members angled to extend forward from both sides of the center plate member, and reinforcing members reinforcing corner portions between the center plate member and the side plate members.
Since the third conventional frame assembly is constituted by the plates as a whole, the third conventional frame assembly has a higher rigidity than that of the first and second conventional assemblies. Further, the reinforcing member of the third conventional assembly is expected to further increase a rigidity of the framework. However, the third conventional assembly has a disadvantage of having a very heavy weight and a disadvantage of having a low comfort. In order to obtain an excellent comfort, it is necessary to support a seat back cushion by zigzag springs as described in the first conventional assembly. An immovable center plate member reduces a comfort.
An object of the present invention is to improve a rigidity of a seat back frame assembly without deteriorating a comfort obtained by zigzag springs.
This object can be achieved by a seat back frame assembly for a vehicle seat which is comprised of a pair of right and left side frame members extending in a vertical direction of the vehicle seat, an upper frame member connecting upper end portions of the side frame members to each other, a lower frame member connecting lower end portions of the side frame members to each other, spring means having an upper zigzag spring and a lower zigzag spring mounted between the side frame members, each of said side frame members having a main plate extending in a front-and-back direction of the vehicle seat and a sub-plate extending to an inside from a rear edge of the main plate, said lower frame member having a center portion and side portions positioned in both sides of the center portion, each of said side portions having a main connection portion extending to an outside from the center portion and a sub-connection portion integrally formed with the main connection portion, an outer end of said main connection portion being fixed to the main plate, an outer end of said sub-connection portion being fixed to the main plate, said sub-connection portion being inclined toward a front upper side so that the sub-connection portion and the lower zigzag spring are at least partly overlapped with each other in the front-and-back direction.